1. Technical Field
The present invention relates to cellular mobile radio. More particularly, the present invention relates to the elimination of interference between adjacent channel competing services in cellular mobile radio markets.
2. Description of the Prior Art
The emerging field of telecommunications referred to as cellular mobile radio provides ready access to mobile telephone services for a great multitude of subscribers at reasonable cost. Such mobile communications must provide reliable, quality information transfer within a given market area, whether or not the market area is relatively flat, such as Cleveland; densely populated with large structures, such as New York City; or mountainous, such as Los Angeles.
U.S. Pat. Nos. 4,525,861 (Zoned Data Communications System for Communicating Message Signals Between Portable Radios and a Host Computer) and 4,545,071 (Portable Radio For A Zoned Data Communications System Communicating Message Signals Between Portable Radios and A Host Computer) describe the basic scheme by which a plurality of channels in adjacent bands are assigned automatically to a plurality of portable radios (mobile telephones) based on channel use and proximity of the portable radios to geographically dispersed transmitter-receivers. Typically, such scheme involves two adjacent 10 MHz bands, each band including 333 channels, each channel spaced 30 kHz apart. Each 10 MHz band is assigned to a competing company that provides services to a local market. One band is assigned to a telephone operating company, such as New Vector (referred to as the wireline service); the other band is assigned to an independent company, such as Cellular One (referred to as the nonwireline service).
Channels are assigned to mobile/portable radios by a central computer to provide optimum use of the channels in each band. The central computer controls a plurality of transmitter/receivers or cell sites within the serviced geographic region. Because such services are generally provided at 835/880 MHz, the propagated signals act more like microwaves than radio frequency signals and are limited to line of sight transmission. This characteristic of the 835 MHz band is exploited to produce a reuseable spectrum by which a given mobile/portable radio is assigned to its nearest cell site on an unoccupied channel. As the portable radio moves from one cell site to another, it is switched to the nearer cell site and assigned to an unoccupied channel, all without interrupting the exchange of information over the portable radio.
One problem inherent in a service operating at frequencies as high as 835/880 MHz is that the signals do not go through hills, tall buildings, or other obstructions. This produces blind spots in the serviced area that may interfere with or entirely interrupt the use of portable radios. One approach to eliminate this problem is to add cell sites to the system. Such additions are costly, running $500K or more per cell site.
A simple solution to the problem of blind spots in a cellular mobile radio system is to add RF repeaters to affected cell sites. Such repeaters are relatively straightforward and inexpensive, running about $50K per site. However, RF repeaters are broadband amplifiers - they must amplify signals in the entire 10 MHz band they service. The broadband nature of the RF repeater makes it very difficult to amplify the entire spectrum of the 10 MHz band without interfering with the adjacent band.
Some repeaters include filters to steepen the skirts at the edges of the amplified band. Simple, effective filters cannot be built for the 835/880 MHz bands. Accordingly, a system operator is faced with the choice of shifting the amplified portion of the 10 MHz band away from the adjacent 10 MHz band and, in the process, losing communications and control channels within his band (and thus limiting the capability of the system), or shifting the amplified portion into the adjacent band (after obtaining permission from a competitor) and tolerating the interference between the channels in the adjacent bands.